Sensing of extremely low concentrations of gases is required for effective detection of various chemical or explosive hazards. The hazardous nature of the compounds of interest makes detection at a safe standoff distance highly desirable. Individual spectroscopic laser techniques for gas sensing, while effective, typically do not provide the sensitivity required for practical standoff hazardous gas detection.
Various spectroscopic techniques may be used in standoff laser detection of hazardous substances. Sensors capable of several techniques simultaneously or cooperatively often yield superior performance results. Among the most useful of these techniques are absorption spectroscopy and Coherent Anti-Stokes Raman Scattering (CARS).
Coherent anti-Stokes Raman spectroscopy (CARS) is a form of spectroscopy used primarily in chemistry, physics and related fields. It is closely related to Raman spectroscopy and lasing processes, but involves a light amplification process that dramatically improves the signal. Two laser beams, one at an excitation (pump) frequency and the second at a frequency that produces Stokes Raman scattering, interact coherently in a sample (e.g., a gas), producing a strong scattered beam at the anti-Stokes frequency. The CARS process taking place in the sample is a third-order nonlinear optical process. The anti-Stokes frequency is resonantly enhanced when the difference in incident laser photon energies coincides with the frequency of a Raman resonance, which provides the intrinsic vibrational contrast mechanism. The anti-Stokes spectra contain information that relates to gas species concentration. The non-linear wave mixing is a vector process, and the laser like anti-Stokes signal leaves the diagnostic volume in a prescribed direction that depends on the vector angles of the pump and Stokes beams. Since the signal is laser like, it can be focused.
Absorption spectroscopy is based on the absorption of photons by one or more substances present in a sample, which can be a solid, liquid, or gas, and subsequent excitation of electron(s) from one energy level to another in that substance. The wavelength at which the incident photon is absorbed is determined by the difference in the available energy levels of the different substances present in the sample; it is the selectivity of absorbance spectroscopy—the ability to generate photon (light) sources that are absorbed by only some of the components in a sample at a specific wavelength—that gives absorbance spectroscopy much of its utility. In one implementation, known as differential absorption, a source laser that is tunable may generate a wavelength that is strongly absorbed by the target, e.g., a gas. Tuning the laser to another wavelength may generate a laser line that is not absorbed. By alternately modulating between the two wavelengths and comparing the ratio of the absorbed to unabsorbed wavelengths, a measure of the concentration of the absorbing gas may be directly obtained.
Differential Absorption Laser Imaging Detection and Ranging (i.e., DIfferential Absorption LIDAR, or DIAL) is a variation of the above technique used in pollution and gas sensing. Two wavelengths of light are used in the same manner as just described, but light is transmitted in pulses and the pulse time-of flight is additionally used to determine distance to the target sample. The light beam is modulated between two wavelengths, one at the absorption wavelength, and a second of a nearby wavelength that is not absorbed. For example, in a gas target, a measurement of the ratio of the pulsed light scattered or transmitted at both wavelengths yields information about the range (distance to the target gas) and concentration of the gas as a function of distance. The range to the gas is determined by measuring the time delay between transmission of a pulse and detection of the scattered signal.
CARS and Differential Absorption spectroscopy are two such spectroscopic techniques which may be used effectively in combination for gas sensing, but these techniques typically require their own individual laser sources and sensors. Typically these two methods require separate laser sources and detectors, with an associated increase in system complexity, volume, and cost.
Practical detection systems using this combination would benefit greatly from the reduction in system size, complexity and cost that would result if the two techniques could be employed using the same laser source.